Vehicular networking and communication is considered as a key technology in the roadmap of the future Intelligent Transport Systems (ITS) because it promises to improve road safety and traffic efficiency. It allows vehicles to exchange information among each other to support various applications such as collision avoidance or cooperative adaptive cruise control (cooperative ACC). Further, using vehicular networking, vehicles can also communicate with roadside infrastructure systems to attain useful information such as roadwork warning, weather warning, or information about available parking in a city.
Currently, vehicular communication is premised on the concept that vehicles and road infrastructure form a vehicular ad hoc network (VANET) in a spontaneous manner to exchange information with each other over the wireless medium. Major effort has been invested in developing ad hoc vehicular communication using short-range communication technologies such as IEEE 802.11 (WLAN). However, a major deployment problem that vehicular communication is currently facing is that it is a cooperative technology, i.e., its effects can be best realized when a certain percentage of vehicles are equipped with this technology. Thus, while there is a great momentum behind the technological development for vehicular communication, the deployment for this system appears rather slow (or stagnant).
An effective bootstrapping strategy for the deployment of vehicular communication could be to rely on the next generation of mobile communication. With the advent of smart phones and the deployment of 4G/LTE mobile communication, the development of mobile high-speed Internet is gaining great momentum and can be helpful for the deployment of vehicular communication. Vehicle manufacturers can equip their vehicles with a smart phone that offers a wide range of applications without relying on the cooperative nature of vehicular communication. On the other hand, smart phones equipped in the vehicles can also be used as a communication device that allows vehicles to exchange messages with each other and/or communicate with the transportation infrastructure.
An important aspect of cellular-based vehicular networking is that there is no direct communication link between a sender and a receiver at the lower layer (e.g., layer 2) such as in the case of WLAN-based vehicular networking. Rather, vehicles have to rely on one or multiple network elements at higher layer (e.g., layer 3 or higher) to relay data. A design approach for cellular-based networking is to introduce a new network element that serves as a message reflector to facilitate the communication among vehicles. This network element functions as a server for the vehicles and road infrastructure, i.e., it processes incoming messages from its clients (vehicles and road infrastructure) and redistributes these messages to them. Since this server is typically responsible for a geographical area, it is termed GeoServer. The main functionality of a GeoServer is to provide vehicles with geographical-related services such as safety- and commercial-related services.
An important application message in vehicular communication is the application-layer beacon message, also called heartbeat, basic safety message, or cooperative awareness message (CAM). This message is periodically broadcast by each vehicle and contains the vehicle's status such as vehicle's position, speed, and heading. This message allows a vehicle to be aware of others in its surrounding. Using the periodic CAM messages received from others, a vehicle can detect potential collisions with other vehicles in its vicinity. The rate at which CAMs are sent needs to be carefully chosen to avoid network congestion and, at the same time, minimize the latency of message passing.
Another important application message in vehicular networking is event-triggered danger warning message (also called decentralized environmental notification message or DENM) that is transmitted by vehicles or roadside infrastructure when a critical situation is detected. Given their importance, DENMs are usually forwarded by the GeoServer to a geographical area in a deployment scenario of cellular-based vehicular networking.
Main issues in vehicular networking include the fact that radio resources are limited but applications need to transmit messages at a high rate. This may lead to congestion in the radio access network and incurs long delay for the messages. On the other hand, applications also have stringent requirement for latency (or freshness) of the received information. These two conflicting requirements pose a difficult problem for applications of vehicular networking.